This invention relates to circuits for reducing acoustic feedback in electro-acoustic systems such as public address systems, telephones, and hearing aids and, in particular, to reducing feedback by the use of digital modulation.
Sound waves are slight variations in air pressure that a microphone converts into an electrical signal of varying amplitude. In a public address system and other electro-acoustic communications systems, the electrical signal is amplified and converted back into sound waves by one or more speakers.
In theory, a signal passes through a system once, never to return. Outdoors and in well designed auditoriums or concert halls, this is essentially true. In other situations, there can be a significant level of acoustic coupling between the speakers and the microphone, e.g. in a "speaker phone." When the output of an amplifier is coupled to the input of the amplifier, one has feedback, a closed loop with the potential to oscillate.
Acoustic feedback in a public address system can cause a mild echo or a self-sustaining ring, depending upon the loudness of the sound returning to the microphone. The cause of the feedback can be poor placement of a speaker relative to the microphone, walls that reflect sound, and/or simply having the volume set too high on the amplifier.
Speakers and microphones introduce system errors that change with each speaker and microphone used because no two components are actually identical even if the components are the same brand and model. For example, substituting one speaker for another can affect the amplitude and phase of the feedback. Changing the placement of a speaker or of a microphone after a system is calibrated can introduce phase and amplitude errors.
There are two difficulties to eliminating feedback in an acoustic system. One difficulty is determining whether the signal passing through the amplifier is from an echo or from an original sound. A second difficulty is determining the travel time of the echo.
U.S. Pat. No. 5,412,734 (Thomasson) discloses an system for eliminating feedback by tagging the original sound with an inaudible replica of that sound, wherein the replica is a frequency modulated (FM) high frequency carrier. U.S. Pat. No. 5,649,019 (Thomasson) discloses a similar system, wherein the replica is a pulse width modulated (PWM) high frequency carrier. The contents of the Thomasson patents are incorporated herein by reference. FM and PWM are essentially analog operations wherein frequency deviation, or pulse width, is directly related to the amplitude of the original signal.
Although FM and PWM are effective techniques, it is desired to improve the efficiency and effectiveness of echo cancellation using an inaudible acoustic tag. Above 20 kHz, the characteristics of acoustic transducers and of a room change significantly with frequency. However, using a bandwidth of 20-40 kHz can introduce distortions due to the change in characteristics between 20 kHz and 40 kHz. A narrower bandwidth reduces the noise in the system and decreases the power required for successfully tagging or marking the original sound.
The ability to cancel an echo depends, in part, upon the ability to faithfully reproduce the original signal, which generally requires a wider bandwidth rather than a narrower bandwidth. Thus, one has the contrary requirements of narrow bandwidth to reduce systemic errors and a wide bandwidth for greater fidelity.
As used herein, "digital modulation" refers to shift keying modulation techniques that are described by the following equation. ##EQU1##
Many such techniques are described, in the context of modulated radio frequency (RF) signals, in texts such as "Digital Communications and Spread Spectrum Systems", Ziemer and Peterson, Macmillan Publishing Company, New York, 1985.
In view of the foregoing, it is therefore an object of the invention to provide apparatus for reducing feedback through the use of inaudible digital modulation of an acoustic signal.
A further object of the invention is to provide apparatus for reducing feedback using a narrower bandwidth than for FM or PWM.
Another object of the invention is to provide a apparatus for reducing noise in the returned, marked signal.
A further object of the invention is to provide apparatus for reducing feedback using less power than for FM or PWM.